In a water scalping tank, employed for classification and reblending of sand or other water-insoluble granular material, a slurry of sand and water is introduced into one end of an elongated tank; the larger particles settle to the bottom near the input end and progressively finer particles settle out toward the opposite (overflow) end of the tank. A series of classification stations for discharge of sand are spaced longitudinally of the tank. Sand is permitted to accumulate to a substantial depth at each station and is then discharged to one or more reblending flumes. The tank may include one flume for blending a controlled specification product and a second flume for an uncontrolled residual product constituting excess sand not used in the specification product. In many instances, the tank has three flumes, which may be used to produce two controlled specification products flumes and an uncontrolled residual product.
The water scalping tank serves three basic functions. One is to remove excess water from the sand or other granular material; the second is to classify the sand into various sizes. The third function is to reblend these different sand sizes in predetermined ratio to meet a quantitative specification. Ideally, the tank would separate the sand into non-overlapping sizes. In actual practice, the sand discharged at each classification station may include several different particles sizes, but there is sufficient differentiation in particle size between the stations to permit reblending within rather closely controlled tolerances.
The most common reblending control for a water scalping tank or similar classifying and reblending apparatus comprises a series of manually adjustable splitter gates, one for each classification station, dividing the output between controlled product(s) and an uncontrolled residual product. However, variations in the gradation of the material fed to the tank may necessitate frequent sampling of the reblended controlled products, followed by manual readjustment of the splitter gates to hold their compositions within specifications. This sampling and readjustment operation is time consuming and wasteful.
One alternative to manual splitter gate control for the reblending operation of a classifying tank provides for multiple outlet valves at each classification station. Timing control is applied to those valves, varying the duty cycles of the outlets to produce one or more controlled products and a residual product. A particularly successful example of this kind of control is disclosed in Keeney U.S. Pat. No. 3,114,479, issued Dec. 17, 1963.
Another quite successful automated control for a classifying tank, of a more sophisticated nature, is described in Cochran U.S. Pat. No. 3,160,321, issued Dec. 8, 1964. In the Cochran system, the specification product is produced in a series of batches. The quantitative ratio between the amounts of sand discharged from the classification stations to each specification product is determined directly by individual timers, one for each product at each classification station; all timers for each controlled product must time out before a new batch of that product is started. The Cochran system also provides an effective tolerance control, utilizing a maximum timer and a minimum timer for each classification station of the tank. A similar batch control, applied to a tank in which each classification station has a single outlet valve followed by a diversion valve for directing the flow to two or more different flumes, is shown in Archer U.S. Pat. No. 3,467,281.
A continuous-operation automated tank control system is described in McCauley U.S. Pat. No. 3,913,788, issued Oct. 21, 1975. In that system, one classification station is designated a master station. Discharge at each station is initiated whenever sufficient material has accumulated to allow a relatively constant flow. Each station has a digital timer for measuring its flow to the specification product; the time of flow for the master station is continuously compared with the flow time for each secondary station in a pre-set ratio that may be different for each secondary station. When the comparison for any secondary station shows excessive cumulative flow from that station, its discharge is diverted from the specification flume to the auxiliary flume, but only until the master station flow catches up. When the comparison for any secondary station shows an inadequate cumulative flow, the master station discharge is diverted to the auxiliary flume until the secondary stations have all caught up.
In any of these different classifying tank controls, considerable difficulties may be encountered if the size gradation of the input to the tank changes substantially, as when a sand dredge moves from one part of a deposit to another. When such a change occurs, the manual splitter gate control or the duty-cycle control of the Keeney patent may no longer be able to maintain the controlled products within permissible tolerances. The Cochran control can still produce the controlled products, but may divert excessive quantities of sand to the uncontrolled auxiliary product. Similar difficulties can occur in the Archer and McCauley systems. In many installations, these problems could be averted if changes in input gradation were identified in a reasonable time.